1. Field of the Invention
This invention generally relates to dye-sensitized solar cells (DSCs) and, more particularly, to a solid-state DSC (ssDSC) tandem module with larger active areas for greater light absorption.
2. Description of the Related Art
FIG. 1 is a partial cross-sectional view of typical DSC structure (prior art). DSCs had typically exhibited low conversion efficiencies until a breakthrough in 1991 by professor Grätzel and co-workers using a nanocrystalline titanium oxide (TiO2) electrode modified with a photon absorbing dye. In modern DSC cells, the photoanode TiO2 electrode is fabricated on a transparent conducting oxide (TCO), a monolayer of absorbed dye on a TiO2 surface, a platinum (Pt) counter-electrode, and an electrolyte solution with a dissolved iodine ion/tri-iodide ion redox couple between the electrode. The structure shown in FIG. 1 has successfully demonstrated an energy conversion efficiency that exceeded 7% in 1991 (B. O'Regan and M. Gratzel, “A low cost high efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature, 353, 737-740, Oct. 24, 1991) and 10% in 1993 (M. K. Nazeeruddin et al., J. Am. Chem. Soc., 115, 6382-6390, 1993). At the present, the highest reported efficiency to date is 11.1% (L. Han et al., “High efficiency of dye-sensitized solar cell and module,” IEEE 4th World Conference on Photovoltaic Energy Conversion, 179-182, 1996).
In order to sensitize the TiO2, a dye molecule is attached to the TiO2 surface. When the dye molecule absorbs a photon, an electron is excited to the lowest unoccupied molecular orbital (LUMO) and is subsequently injected into the conduction band of the TiO2. As a result of this, the dye molecule is transformed to its oxidized state. The injected electron percolates through the porous nanocrystalline structure to the TCO (negative electrode, anode) and finally through an external load to the counter electrode (positive electrode, cathode, and Pt). At the counter electrode, the electron is transferred to tri-iodide in the electrolyte to yield iodine (I3−+2e−→3I—). The cycle is closed by reduction of the oxidized dye by the iodine in the electrolyte.
Another type DSC uses solid state hole transporting material (ssHTM), instead of an electrolyte solution, to complete the cell fabrication. The common ssHTM includes inorganic p-type semiconductor (e.g., CuI, CuSCN, or CsSnI3), organic p-type semiconductor (e.g., spiro-MeOTAD), ionic liquid electrolytes, and polymer electrolytes. A review of recent progress in solid state DSC can be found in B. Li et al., “Review of recent progress in solid-state dye-sensitized solar cells,” Solar Energy Materials & Solar Cells, vol. 90, pp 549-573 (2006).
The most frequently explored strategy for achieving higher efficiency in solar cells has focused on the use of a tandem cell structure, through which individual cells can be tuned to a particular frequency of the spectrum. This allows the cells to be stacked such that layers capable of capturing shorter wavelengths are located on top, while longer wavelengths of light are allowed to pass through the top and travel to the lower layers. For DSC cells, several tandem cell concepts and structures have been proposed. One proposal suggests a random mixture of two or more dyes with different absorption spectra (molecular cocktail). So far, this approach has not led to higher efficiency cells when compared to the best (single) dye with broad absorption characteristics.
FIG. 2 is partial cross-sectional view depicting a DSC made with two separate layers of photoanode sensitized with different dyes, as described by Chiba et al. in U.S. Pat. No. 6,677,516 (prior art). In this case, one layer of TiO2 contains magnesium oxide on the surface. Through surface etching, the dye molecules attached on the particles within this porous layer are removed together with the magnesium oxide layer, which can subsequently be replaced by another type of dye molecule(s). Although both the absorption spectrum and output current are improved, the output voltage is still limited to the TiO2-electrolyte energy level alignment. As a result, the overall output efficiency is still lower than the 11% obtained using single dye system.
FIG. 3 is a partial cross-sectional view of a DSC where two photoanode layers are independently fabricated on separate substrates and then combined together (prior art). The first cell contains the first paste and the first dye fabricated on a substrate while, separately, a second cell containing a second paste and second dye is fabricated on a second substrate. Next, the two cells as bonded together by using a sealing agent in a manner such that they oppose each other with a platinum mesh or a platinum coated carbon mesh between them. An iodine charge transport layer is deposited between the two electrode pastes. Electrically, the two solar cells are connected in parallel, so the output voltage is ultimately controlled by the lower one, while the output current is the sum of the two. Since the electrolyte is the same and the electrode pastes have similar band structures, the output voltages of the two cells are very similar. The challenges of this cell are: (a) the middle electrode (cathode) has to be transparent in order to allow light to penetrate to the bottom cell, and (b) an effective connection of the three external electrodes (two anodes, one cathode) in a small area.
FIG. 4 is a partial cross-sectional view depicting two DSC cells stacked together (prior art). The DSC cells are fabricated separately, and both are sealed before being brought together. The two cells can be connected either in parallel or in series. The challenges for this cell are similar to the previous structure: (a) the Pt has to be thin to be semi-transparent, and (b) the electrical connection cannot occupy too much of the area. In addition to these challenges, the cost of this cell is basically twice that of a single cell.
FIG. 5 is a schematic drawings of a tandem DSC structure with two external electrodes and its corresponding band diagram and charge flow (prior art). The first semiconductor electrode functions as a hole transport material and the second as an electron transport material, whereby the two potential differences between the redox potential of the electrolyte and the two active electrodes sum up to the photovoltage. The photocathode is made of NiO nanoparticles for hole conduction and the photoanode is made of nanoparticle TiO2 for electron conduction. The electrolyte functions as the electrical connection for the two DSC cells. The difference between this cell and the previous two tandem cell examples is that no Pt electrode is needed between the two cells. In the previous two examples the two nanoparticle electrodes are photoanodes. The challenges of this cell are: (1) the current generated in these two cells has to match well in order to obtain the maximum output current since the two cells are connected in series, and (2) the voltage output is limited because the electrolyte is used to connect these two cells. For the example shown in FIG. 5, the NiO cell produces a voltage output of only ˜0.2V.
FIGS. 6A and 6B are, respectively, a schematic diagram and partial cross-sectional view of an integrated Z-contact DSC module (prior art). The Z-contact modules consist of two opposing electrodes, with the connection between neighboring cells through a conducting medium. In order to prevent the conducting grids from iodine ion decay, a sealing barrier is needed. The advantage of Z-contact modules is that the TiO2 paste is printed at one panel, while the disadvantages are a small active area, due to the three layers needed between the cells (two seal layers and one interconnect), and a lower fill factor resulting from the series resistance of the conductor. Note: solar cells are often represented by the parallel connection of a diode and a current source. For simplicity, (just) a diode symbol is used in the drawings to represent a solar cell.
FIGS. 7A and 7B are, respectively, a schematic diagram and partial cross-sectional view of an integrated W-contact DSC module (prior art). The W-contact module avoids the above-mentioned Z-connect interconnect problems. However, neighboring cells are alternately biased, while still requiring separation of the cells using an effective seal. The advantage of this design is a simple structure due to the omission of the interconnections. Hence, a higher fill factor can be expected. However, the disadvantage of the W-contact modules is the necessity of maintaining all cells at an almost identical JSC, which is a problem since two different types of cells exist, one illuminated from the TCO side and the other illuminated from the Pt side.
It would be advantageous if a solid state DSC module existed with a tandem cell design that increased light absorption and cell efficiency, and used a simple module connection with no sealing issues.